1. Field of the Invention
The present invention relates generally to germicidal lamps, and more particularly the invention relates to such devices used in air and surface sterilization.
2. Description of Related Art
Germicidal lamps have been known for quite a while. Typically, a germicidal lamp has two primary parts, a tube and a base (or fixture). The tube is typically a long, thin sealed hollow cylinder containing a discrete amount of inert gas and liquid mercury. These tubes are made in standard sizes and power capabilities, and have standardized outputs. Standard tubes are made by General Electric (G25 T8 and G36 T6), Sylvania and others. These standard tubes have lengths of 18 or 36 inches.
The fixture of a germicidal lamp is designed such that the tube may be easily installed and removed, while holding the tube firmly during other times. The fixture typically also incorporates a ballast. The ballast is an electrical component that converts a standard power input (e.g., 117 V, 1 A 60 Hz AC household current) to a form appropriate to the tube. Typically germicidal lamps use the same standard magnetic ballasts as used by fluorescent lamps. These ballasts deliver electricity of 400 V, 420 mA and 60 Hz to the tube.
Germicidal lamps typically are available in standard form factors. The standard tubes described above fit into a standard base, which is 18 or 36 inches long. The ballast is disposed at one end of the fixture inside a casing, and the fixture has sockets at either end to provide mechanical support for the tube and electrical contacts. The typical fixture is a shiny metal such as aluminum or steel.
Germicidal tubes differ significantly from electric discharge devices used in ultraviolet gas spectroscopy (VUV tubes). Germicidal tubes are low pressure and emit ultraviolet light at the primary and secondary emission lines of mercury--254 nm and 187 nm. In contrast, VUV tubes are high pressure tubes, operate at very high temperatures and emit light at different spectra.
Typical germicidal lamps create ozone. Along with mercury's primary emission at 254 nm, there is a second, but frequently large amount of radiation at 187 nm. This secondary emission results in ozone creation. In the typical application of a germicidal lamps, this ozone is desirable as a germicide.
One common use of germicidal lamps is in water treatment. In water treatment, ozone is generally considered very desirable. However, the ozone in these water treatment systems is very controlled. The ultraviolet tubes are typically enclosed in quartz glass sleeves which restrict air flow, thereby elevating ambient temperatures. This is necessary because the water would otherwise draw away heat from the tube, shutting down the tube.
Although it has long been known that germicidal lamps may be used for air sterilization, their actual implementation has been fairly limited. One reason for this is the ozone creation already discussed. Ozone is harmful to humans, and is a known carcinogen.
Another reason that germicidal lamps have not been use for air sterilization has been skin-effect cooling. According to several publications, a germicidal tube operates best at temperatures well above room temperature, and include heaters to vaporize the liquid mercury.
In skin-effect cooling, air moving across the outside of a tube reduces the normal operating temperature. As the temperature drops, the mercury vapor pressure drops, leading to a drop in ultraviolet output. It is known that if the ambient temperature drops below 72.degree. F. or air is blown over the tube, ultraviolet output of standard germicidal tubes drops at an extremely rapid rate--as much as a 75% depreciation of output at approximately 58.degree. F. Furthermore, the tubes age quicker at low temperatures.
Because of skin-effect cooling, 2 to 4 times as many standard germicidal lamps must be used in low temperature applications to maintain output, and the tubes must be replaced more often--every 2500 hours. The high cost of extra tubes and the maintenance costs have limited the practicality of germicidal lamps.
There have been numerous attempts to use standard germicidal lamps in operating rooms and hospitals. However, higher output tubes used in these applications produce large amounts of ozone, which as mentioned, is undesirable. This limits their applicability. Furthermore, the germicidal lamps have been used in the upper reaches of rooms, thus limiting their utility.
Despite the clear benefits of germicidal lamps, problems such as ozone, depressed output in low temperatures and short tube life have prevented their use in all but highly specialized applications. One area which has long sought to use germicidal lamps is the heating, ventilation and air conditioning (HVAC) industry. HVAC systems typically comprise fans, ducts and valves for moving air where needed. An HVAC system may also include a cooling plant and a heating plant for, respectively, cooling and heating the air. In most systems, air is drawn in, filtered, cooled or heated, humidified, and then delivered to a room. Air is also drawn from the room for recirculation through the HVAC system. The HVAC industry has adapted standards for the rate of air exchange in a room, as well as standards for introduction of fresh air into an otherwise closed HVAC system.
Although numerous companies have attempted to provide germicidal lamps for HVAC, the standard germicidal lamps have been unsatisfactory. Typically, a standard germicidal lamp is installed in the HVAC system or in a room where the air is still or warm.
Typical germicidal lamps cannot operate in the harsh environments of HVAC systems. HVAC systems typically are used for controlling the temperature, humidity and comfort of the air in various spaces. Such spaces include offices, living spaces, commercial spaces, industrial spaces, and the interior of vehicles such as cars, buses, planes and ships. The HVAC systems necessarily deliver air which has a temperature, humidity and flow rate which are deleterious to germicidal lamp operation, and the frequent changes of temperature, humidity and flow rate further destabilize these standard germicidal lamps.
More recently, the HVAC industry has also focused on indoor air quality (IAQ). In addition to temperature, humidity and comfort, IAQ addresses the components of air and how the air's components may be controlled to enhance comfort, health and other needs. Often, the goal of HVAC systems is to provide especially clean air, be it for semiconductor or pharmaceutical processing facilities or hospitals and homes.
One reason that fresh air is introduced is because of the existence and growth of molds, spores, bacteria and viruses in the HVAC system. However, introducing fresh air and increasing air exchange rates requires bigger ducts, more powerful fans, more powerful heating and cooling plants, and a lot more power to operate these.
Where it is attempted to install standard germicidal lamps in an HVAC system, installation requires human access to the interior of the duct and typically requires the cutting of an access door or a large opening for the fixture. Furthermore, standard 18" or 36" germicidal lamps are either too large or too small for HVAC ducts. Use of these lamps results in either poor UV coverage of the target area, or the use of any lamps with over-coverage. With over-coverage, installation, usage and maintenance costs rise. To provide adequate ultraviolet dose with low output lamps, bulky and expensive racks have been proposed. Yet, these racks nonetheless require significant engineering and manufacturing efforts.
It is therefore the object of the invention to provide a germicidal lamp suited for use in HVAC systems. Such a germicidal lamp would be easy to install and service, and be able to operate for long periods within the HVAC system and not generate appreciable amounts of ozone. These objects and others are provided by the of the present invention.